Geotechnical Engineering and Design

Geotechnical engineering is the subsurface study related to the construction of a structure such as a building, highway, or bridge.  Geotechnical drilling and sampling forms the basis for the overall geotechnical investigation and engineering report.  It is usually focused on evaluating specific site conditions related to soil stability and load-bearing capacity of materials at the proposed site.  The investigation is generally completed prior to, or as a part of the preliminary and final design steps for the structure.

What you find under your building site can be as important as what you build on it.  Geotechnical engineering is a branch of civil engineering that deals with the engineering behavior of earth materials, including soil, rock, and groundwater. The study investigates subsurface conditions and materials, such as landslides, non native fill, fissures, soil erosion, and faults. ACS geotechnical engineering can drill, sample, test, characterize and prepare a report on your site’s earth materials. The client can then predict the reaction to load conditions and other factors that can influence construction methods, costs, times and potential delays.

ACS works with builders, architects, engineering companies, design-build contractors, departments of transportation, home owners and municipalities.

Geotechnical capabilities include:

  • Investigation of Existing Site Conditions
  • Evaluation of Subsurface Conditions
  • Recommendations for Mitigation of Expansive/Collapse Potential
  • Conventional and Post Tension Foundation Design Parameters
  • Subgrade Preparation Requirements
  • Trenching and Backfill Requirements
  • Construction Inspection and Pad Certification
  • Excavation Boring Depths of up to 100′
  • 4×4 for Hillside, flood plain, and remote or difficult environments
  • Forensic drilling for pad, slab and environmental issues
  • Shallow spread foundations
  • Drilled shaft foundations
  • Mat foundations
  • Caisson or Pile foundation systems
  • Pavement thickness and design
  • Building slab systems
  • Earth retaining systems
  • Slope stability
  • Septic percolation testing
  • Retention percolation testing
  • Double-ring Infiltrometer
  • Floor level surveys
  • Earth fissure studies
  • Soil corrosivity potential
  • Rock anchor systems
  • Import soils recommendations
  • Boulder stability
  • Seismic Refraction Analysis

Field Work

Drilling and Sampling

ACS has 4 drilling rigs (CME 45, CME 75 and Mobile B59) and the crews to provide services statewide in Arizona.  Based on client information, ACS can determine a time frame for mobilization and prepare a drilling, sampling and testing plan for the project.  Depending on circumstances, before mobilizing the site is ‘blue staked’ for any underground hazards.  For special circumstances such as work hours, site accessibility, noise issues, traffic issues, code compliance etc. ACS is flexible and can generally accommodate the client.

ACS owns and operates 4 drilling rigs (CME 45, CME 75, CME 750 and Mobile B59). ACS drilling capabilities include AUGER, OVERBURDEN, ROCK CORING and DIRECT PUSH. The CME 750 allows accessibility to rough terrain.

Seismic Refraction Analysis

ACS conducts seismic refraction surveys in support of its geotechnical investigations.

From the Surveys ACS can:

  • Determine the approximate depth to a bedrock layer
  • Evaluate rock properties based on the seismic wave velocity of the rock

Seismic surveys use acoustic energy put into the ground at a source location. The energy supply may include a sledge hammer striking a metallic plate, a weight drop or a type of explosive source. The seismic waves propagate into the earth at a velocity dependent upon the density and elastic properties of the ground materials. When the waves reach a change in the interface (intersection of two layers), the layer velocities may change significantly and some of the energy is reflected back to the surface, along the interface (refracted) and is transmitted into the lower layer.

Refraction occurs where the velocity of the lower layer is higher than that of the upper layer, generally the lower layer being a more dense material. Refracted waves travel along the interface at the velocity of the lower layer and continually transmit energy back to surface. Receivers or ‘geophones’ are laid out in a line at an even spacing on the surface, they record the quicker arriving incoming direct and refracted waves and the slower arriving reflected waves. Seismic refraction involves the analysis of the travel times of the first energy to arrive at the geophones. These arrivals or ‘first breaks’, are from two sources 1) the direct wave from the source itself normally at geophones close to the source, or 2) refracted waves at geophones further from the source.

The refracted energy waves arriving at surface are carrying geological information.  All of the waves registering at the geophones are filtered and the refracted waves are interpreted to create a cross sectional view of the sub-surface.

Soil Resistivity Analysis Using Wenner Array

ACS provides resistivity measurements using a Wenner Array in the field.

A Wenner Array resistivity survey measures the ground’s capacity to pass an electrical current. The data is used for designing earthing systems for towers, substations or for structures which need specialized grounding and for measuring the corrosion susceptibility of buried pipelines, tanks, piers and other steel structures. Four equally spaced electrodes are set in a fixed configuration.   A low frequency current is applied across the two outer electrodes and the voltage measured across the inner electrodes. The voltage data is used in calculations to determine a value representing the average resistivity of the ground between the electrodes. Resistivity depth readings are acquired by keeping the array centered on the same point and making electrical soundings using an expanding electrode. The depth penetration of the readings is directly proportional to the spacing of the electrodes.

Laboratory Work

ACS has an AASHTO R18 accredited laboratory in Soil, Aggregate, Hot Mix Asphalt and Concrete testing.  We are also Arizona Department of Transportation, ADOT accredited.

Reporting

Based on the discussions with the client and local requirements ACS prepares a soils report which addresses the issues needed to proceed with a project.